We find this circuit in an Electronics Now. The indication of the logic level is done on a 7-segment display according to the connections given in the diagram.
The circuit shown is from the Texas Instruments optoelectronics manual, applying to low power...
This circuit was found in a 1968 General Electric documentation. It has an input impedance of 2 M ohms...
This filter that rejects a frequency range determined by the double-T was found in the August 1993...